Wheatstone bridge



Aug. 15, 1933. R. A. KOPP WHEATSTONE BRIDGE Filed Feb. 20, 1929 Patented Aug. 15, 1933 WHEATSTONE BRIDGE Reinhold A. Kopp, Chicago, Ill.,. assignor to W. M. Welch Mfg. Co., Chicago, Ill.,

ration of Illinois a Corpo- Application February 20, 1929. Serial No. 341,529 3 Claims. (01. 201-62) The present invention relates to a Wheatstone bridge, and has particularly to do with a simplified construction thereof and to the skeleton-i izing of the base thereof, also, to the provision of an improved key member.

The fundamental construction of a Wheatstone bridge is well known and consists of:

(1) A high resistance wire, which should be called the bridge wire, either exactly one meter or one-half meter in length, depending upon the form, stretched between two contact points and lying fiat on some form ofvmillimeter scale, which is mounted securely on the frame.

(2) A conductor, running parallel to the bridge wire and of such length and mounted in such position that gaps of approximately two inches shall be made between the ends of this conductor and the ends of the stretched wire or the contact plates to which this wire is attached. Binding posts are so disposed on the contact plates. of the resistance wire and at the terminals of the long conductor so that two resistances, one known and the other unknown, can be placed across these gaps.

( 3) Some form of key, which is supported over .the bridge wire;and is free to slide from one end to the other. This key may at all times make contact with a second long strip screwed to-the base alongside the bridge wire or else a galvanometer' wire may be connected to this key byubinding posts, in which case the key can slide on themeter scale itself without other contact.

By means of this instrument, as described, together with a suitable disposition of battery and galvanometerand connecting wires, the set-up of the Wheatstone bridge is made complete.

Heretofore, Wheatstone bridges of the type used laboratories, have comprised rather bulky devices the parts of which have been adapted to be assembled upon a relatively broad wood or other panel. The scarcity of suitable wood, and the inability to obtain good wood-workers, has made it imperative that a change be had in the construction of these devices. The present invention is believed to solve all of the problems heretofore presented in the construction of Wheatstone bridges, and to minimize the amount of wood. and woodworking which is re- (mired in connection therewith.

Among the objects, ,of the present invention,

are:

A simplified. arrangement of the parts of a Wheatstone bridge to be used for laboratory purposes.

An improved Wheatstone bridge having a skeleton body.

Improved means for mounting the conducting parts of a Wheats tone bridge.

An unique arrangement of frame members for the formation of a Wheatstone bridge.

An improved contact switch or key adapted to be used in connection with a Wheatstone bridge.

These objects, and hereinafter appear, construction, unique such other objects as may are obtained by the novel arrangement, and improved Figure 2 is a sect Figure 1;

Figure 3 is a detail, in perspective, 'of one of the end members vice, intermediate sections being broken'away;

ion along the line 22 of of the device, some of :the

parts generally attached "thereto being removed; Figure 4 is a transverse section in the line 4-4 of Figure 3;

Figure 5 is a plan View of the spacer or middle supporting member; and Figure 6 is a plan view of the blank for forming the switch being broken away.

or key member, sections Like reference characters are used to designate similar parts in the drawing and in the description of the invention hereinaftergiven.

From reference to Figure 1, it Will be noticed that instead of the usual wood panel, the device is mounted upon two similar, end members which arereversed, and upon a spacer or sup porting member which may be omitted if desired.

While the spacer is of utility in bridges of one meter dimension, it is generally omitted in the half meter bridges.

In another type of bridge, of either .half or one-meterlength, two of the spacer members may be used. The device consists of two supporting members, 10 and 11,

of high insulating material and of special shape to which are connected two parallel, conducting members, 15 and 24, ton frame.

thus forming 'a rigid skele- If the length or special character of the bridge requires it, a third, 1'7, and possibly a fourth,insulating member maybe disposed between the two end members to provide extra support or to aid the placement of resistance units for the proper action of the device. Each of the end m embers 10 and 11 is shaped like the small letter h, one being reversed, with the short leg 12 opposed and inwardly directed. Said members 10 and 11 are adapted to be made from moulded bakelite, or its equivalent, and are hollow as shown in Figure 2 and subsequent figures only the long leg 13 of the h being provided with feet 14. These members, by their shape, provide the gaps hereinabove referred to at each end of the bridge, and with the center member (or a plurality thereof 1 may provide secondary gaps for, special purposes.

Intermediate the two short legs 12 .of the h members 10 and 11 and secured thereto by fastening means comprising binding posts 16 is the conventional polished brass bar or a bar of other material 15 the electrical resistance of which is slight. The electrical resistance of this member 15 is generally considered or is actually negligible.

. At each end is the binding post 16 which is adapted to extend through the material of the short legs 12 of the h members 10 and 11 to provide a firm fastening means for the end of the bar 15. I

Said bar 15 is secured to a central supporting member 17 of a U-shape and also of balkelite and hollow, by two screws 18 passing through the bar 15 and through the material of said supporting member, to be secured in the hollow section thereof by a nut.- Projecting upwardly and, of course, through the bar 15 intermediate the two holding screws 18 therefor is a binding post 19. This is the binding post which is used to make one of the. electrical contacts with a galvanometer, not shown. The open endof the central supporting member 17 is directly under the binding post 19 just referred to.

Disposed upon the upper surface 'of the short leg 12 of the h-shaped members 10 and 11 is a metric stick or other measuring stick 20 generally of wood, or of other insulatory material, which has extending over its entire length a hig h resistance wire 21. Such stick is usually rectangular in cross section. (See Figure 2.)

At the top of the long leg 13 of the h members l0 and 11 are other binding posts, 22 and 23, which are electrically connected to a tube or circular rod 24 extending the entire length of said device, and upon which the switch or key member hereinafter to be described is disposed. Upon each of the end members 10 and 11 is also an L-shaped conducting, strip 25 and 26, the end of the shorter leg thereof being turned upwardly at 27 and 28 to provide an end rest or shoulder support for the metric stick as well as a definite contact point for the bridge wire 21. Said L-shapedcontact bars 25 and 26 are held in position by screws 30, which facilitates the attachment of the high resistance wire 21 to the contact bars, 25 and 26, and by a binding post, 32 and 33, adjacent thereto extending therethrough, such binding posts 32 and 33 being at the lower ,end of the long leg 13 ,of the h members 10 and 11. The metric stick 20 is held in position by a screw 34 which is adapted to pass upwardly through the material of the endmembers 10 or 11 thereinto.

Thus two pairs of adjacent binding posts are provided, 16 and 32, and 16 and 33. Between one gap aknown resistance is disposed, and between the other gap the material whose resistance is to be determined iselectrically disposed. By'the usual arrangement of batteries, the two electrical circuits for this type of work are'provided with current, there being'necessary to complete the galvanometer circuit a wire contacting with binding post 19 and an electrical connection between the round rod 24 and the high resistance wire 21. electrical connection with the said galvanometer circuit may be had at either end thereof through binding post, 22 or 23.

This connection with wire 21 is in theform of a switch or key member 36 comprising a fiat sheet of spring metal, which has a centrally enlarged .portion 37. The material adjacent the edges of such central portion is cut out so as to provide two pairs of opposed fingers 38 which extend almost around rod 24.

Between these fingers is the undersection of the key and also the oversection thereof. undersection 39 is substantially straight except whereit is slightly bent to provide a fiat portion 40 which is adapted to slide freely along the edge of the metric stick 20 free from contact with the high resistance wire 21 disposed longitudinally thereof and about the longi-, tudinal center thereof.

Thevupper section has one section 41 pointing upward, a second section'42 substantially horizontal when in normal position, and a third section 43 comprising a downwardly directed part having at its extreme end an outwardly pointed tip section 44 providing a pointer 45 facilitating the reading of a scale upon the metric stick 20.

Adjacent the end of the major horizontal portion 42 is an aperture 46, through which the shank 47 carrying a V-edged circular contact 48 is inserted. A threaded handle 49 made of insulating material is secured to contact 48 by attachment to shank 47 and is so adjusted that the horizontal edge of the V-edged portion 48,

is adapted to always be disposed transversely 1:15

Of course,

The

tained through said contact" or key member, i. e.,' I

the upper leg of the switch member just described. Stateddiiferently, the connection with wire 21 is in the form of a switch or key member 36, comprising a flat metal stamping of such shape and so disposed that it ordinarily fitsloosely' about the tube 24, and is capable'of sliding easily from end to end of tube 24. In shap ing the metal stamping, one part'of it, '39 -'-Fig. Y 2, is so bent and disposed as to rest upon the edge of the meter scale. The other part, 42, is

disposed just above the bridge wire and carries an insulating button, '49, 'with a sharp contact piece 48 on its under side. As these two members, 39 and 42, are parts of the same piece of phosphor bronze or other spring imaterial and i as part 39 rests normally upon the meter'scala,

not depressed is capable of sliding with loose contact to any point onv tube 21, but at the moment of depression becomes rigidly in contact with tubular bar 21 and instantly provides firm electrical contact necessary for the proper ac tion ofthe device. I a

As arranged, the central portion 3'7 of the device fits the rod 24 loosely when the key is in the dotted line positionof Figure 2. When pressed downwardly, so that the contact bar 48 comes into contact with the high resistance wire 21, the flexing of the spring material of the key caused thereby forces the fingers 38 more closely about said rod 24 so that when in this position, the key which in its open position slides freely along said rod, grips said rod and is held against displacement therealong.

The flexing of the spring material is brought about through the force manually imparted to the top section of the key which causes such portion to approach the under section thereof which section is held against displacement after its contact with the top surface of the metric stick 20.

The conductor 15 referred to herein above is rigidly connected to the bakelite end pieces 10 and 11 so as to form one side of the frame or skeleton. The other side of the frame or skeleton, which is a metal tube or rod 24 will also be fastened to the arms of bakelite end pieces 10 and 11 projecting beyond the gap, the whole arrangement forming a rigid frame. The bridge wire 21 will be stretched between the contact plates 25 and 26 provided on the top surface of the bakelite end pieces 10 and 11, the wire resting upon the points 27 and 28 and upon the meter or half-meter scale 20, depending upon the form provided.

The metal tube or rod 24 which forms one side of the frame carries the special form of key described which is formed around the tube and is capable of sliding from one end to the other. This key, which is really a sleeve, is slit on one side and has two metal tongues, one fastened to the under side of the sleeve, and the other to the upper side. When the sliding key is in position, the tongue fastened to its under side, rests upon the meter stick 20, while, the upper tongue which is so disposed as to have a button or handle near its end with a wedge-shaped piece of metal directly under the button, takes a position directly over the wire 21, but does not touch the wire except when the button is pressed downward.

The extreme end of the tongue is pointed in.

' stick directly underneath the bridge wire.

The particular point to be observed is that this metal sleeve, when the key is not pressed, is in loose electrical contact with the metal tube 24 which carries it, but when the upper tongue is pressed downwardly the lower tongue is held by the scale. This tends to squeeze the two parts of the sleeve together so as to make the contact with the metal tube electrically perfect. The moment the finger is removed the loose contact is restored and the key or sleeve without very much friction.

can be moved to any other part of the scale While it is believed that the preferred form for the end members 10 and 11 of the device is shown, these members may be materially altered without departing from the subject matter of the invention which is to provide a skeleton support for the various parts of a Wheatstone bridge. Likewise, the key member, the subject matter of which is the provision of a loosely ad-' justable member in open position which is firmly held when in contacting position, may be materially altered without departing from the spirit of the invention voiced in the claims appended hereto.

I claim:

1. A base for a Wheatstone bridge of the slide wire type, and comprising end members of a generally U-shaped configuration in cross section and of highly insulatory material, said end members having a top plane surface simulating the configuration of the small letter h, in combination with a plurality of parallel members, intermediate said end members, said parallel members including a metal conductona graduated stick of selected length, a high resistance wire over such graduated stick, and a conductor of tubular form, there being a key on said tubular conductor and slidable there along.

2. In a structure for a Wheatstone bridge and comprising a graduated stick, a resistance wire in juxtaposition thereto, and a bar, a slide contact on said bar comprising a strip of spring metal bent near its middle to-form two legs loosely surrounding and engaging said bar and being supportedthereby, the lower of said legs normally resting upon said graduated stick, and the upper leg having an insulating head and a knife edge thereon and being positioned to contact said resistance wire at any desired point along the length of said bar when pressed in the direction of said lower leg, the pressure upon said upper leg bringing the two legs of said contact together whereby to tighten and make secure the otherwise loose engagement of said contact with said bar.

3. A contact key for use on a rod in wire device adapted to form a part of a Wheatstone bridge, or resistance wire measuring instrue' mentality, and comprising a graduated stick and a slide being mounted upon said slide rod and compris-.

ing a strip of spring metal bent near its middle to loosely surround said rod to be supported thereby, the ends of said strip forming legs, the lower of said legs resting normallyupon said graduated stick, and the other leg being at an acute angle to the lower leg, force upon the upper leg decreasing the angle between said legs, to cause the main body of said strip' to grip said rod firmly. i

- REINHOLD A. KOPP. 

